Bathroom tissue must reconcile several conflicting properties: bath tissue must be strong, soft, flushable, dispersible and degradable. Even achieving desirable combinations of just these properties at an economically viable cost is a considerable challenge. However, even though a bathroom tissue which could be premoistened and used wet would provide significant new benefits to the user in regard to both extra cleaning and a feeling of freshness, no product currently on the market is really well suited to be used premoistened.
While at least one brand of commercially available bath tissue possesses some degree of wet strength, it appears that the manufacturer's purpose in including temporary wet strength in those products may be to counter the effects of wetting often occurring during normal use. When attempts are made to use these tissues after premoistening, the tissues "shred" and "pill" quite severely. Thus, rather than providing enhanced cleaning, attempted use of these products in a premoistened condition often leaves considerable detritus of shreds and pills of paper on the area that was to be cleaned, thereby largely defeating the purpose of attempting to use tissue premoistened.
However, adding resistance to wet abrasion as an additional conflicting required property to those previously mentioned poses an even tougher technical challenge. Construction of a tissue which has sufficient wet strength that it can be used premoistened inherently conflicts not only with flushability and dispersibility but also with retaining sufficient softness to be used either premoistened or dry. Nevertheless, the present invention provides a tissue which (i) has sufficient wet strength and resistance to wet abrasion that it can be used premoistened; (ii) is flushable; (iii) is dispersible and biodegradable; (iv) has dry strength comparable to premium bath tissue; and (v) has softness comparable to modern premium bath tissue.
The tissue of the present invention reconciles these conflicting objectives by providing a tissue having a glabrous surface coupled with an initial normalized temporary wet strength of at least about 24-25 g/in, preferably about 35 grams/inch as measured using the Finch Cup method for an 18.5 lb/3000 sq ft ream, the tissue exhibiting a wet-to-dry CD (Cross Direction) tensile strength ratio of at least about 18%, preferably over 20%. Temporary wet strength is provided by use of temporary wet strength resin while in many cases softener/debonder helps bring the wet-to-dry ratio into the desired range and prevent the dry strength of the tissue from being so excessive as to unduly degrade the perceived softness of the product.
Simply adding a quantity of temporary wet strength resins such as cationic aldehydic starches to conventional furnishes for tissue does not guarantee that the product will be well suited for use premoistened. The present inventors have found that unless the tissue has both a glabrous surface and a normalized CD wet tensile of at least about 25 g/in, preferably 35 g/in, as measured by the Finch Cup Test ("FCT") at a basis weight of about 18-19 lbs/3000 sq ft ream, the tissue will typically pill or shred when an attempt is made to use it premoistened.
We have found that once the absolute (not-normalized) CD wet tensile of each sheet drops to about 12 g/in or less, the sheet does not usually have sufficient integrity to survive normal use when wet even though the sheet may not pill if handled gingerly enough to avoid tearing the sheet. Throughout this application, where a normalized wet tensile strength is mentioned, it should be understood that the tensile strength is as determined using the Finch Cup procedure in which a 1 inch sample of converted ready-to-use product having a basis weight 18.5 lb/3000 sq ft ream, (single ply or multi-ply as the case may be) is clamped in a special fixture termed a Finch Cup, then immersed in demineralized water at neutral pH and tensile tested at the indicated time after immersion. For initial wet tensile strength, the measurement is conducted 5 seconds after water is added to the cup. We prefer use of this procedure as we have found that the results obtained using the Finch Cup Test ("FCT") are reasonably reproducible.
Since the critical factor with regard to formation of pills seems to be the degree and strength of the internal bonds between the fibers in the sheet, for basis weights other than 18.5 lb/3000 sq ft ream, the critical tensile strength values (25 g/in or 35 g/in and so forth, as the case may be) should be adjusted proportionally to the basis weight i.e., normalized. For example, a 9.25 lb/3000 sq ft ream sheet having a CD wet tensile of about 17.5 g/in will perform satisfactorily as the CD wet tensile is proportionally the same as a 18.5 lb/3000 sq ft ream sheet having a CD wet tensile of 35 g/in and, accordingly, the normalized CD wet tensile of this 9.25 lbs/3000 sq ft ream would be 35 g/in. This squares well with our experience in which single plies of 9.25 lbs/3000 sq ft ream tissue have been satisfactory at CD wet tensile strengths of 22 and 16 g/in, while single plies having a CD wet tensile of 12 g/in fail by shearing without leaving pills.
To ensure that the tissue product will be sufficiently flushable to avoid requiring an excessive number of flushes to clear the bowl, we prefer that the wet strength of tissues of the present invention decays rapidly, exhibiting a normalized cross direction wet tensile of less than about 2/3 the initial value when measured 30 minutes after immersion, and ultimately dropping to about 15 g/1" strip after immersion for over about 10 hours.
Simple addition of a temporary wet strength agent often produces a paper product that does not possess sufficient softness to be acceptable as a premium bathroom tissue for normal household use. To help bring the softness of the sheet into the premium or near premium range, we have found that it is desirable to vary the jet/wire ratio to make the sheet a little squarer than we normally use in production of wet pressed tissues. For example, in production of conventional wet press tissue, we normally control the jet to wire ratio so that the ratio of machine direction dry tensile strength to cross direction dry tensile strength of the basesheet (before converting and embossing) is about 2.5.
For tissues of the present invention, we prefer to use a jet to wire ratio producing a base sheet having ratio of MD dry tensile to CD dry tensile of less than about 2.2, more preferably from about 1.6 to 2.1, most preferably from about 1.8 to 1.9. Similarly, we prefer to impart slightly more crepe to the web than we would normally use. For example, in conventional tissue, we would normally impart about 18-20% crepe to the web as it is creped off of the Yankee. For the tissues of the present invention, we prefer to impart a crepe of at least about 22%, more preferably at least about 23-24%.
1. Field of the Invention
The present invention is directed to a soft, strong, flushable, dispersible and biodegradable paper product having temporary wet strength which may be premoistened before use and resists pilling and shredding when used premoistened.
2. Description of Background Art
In order to provide a household bathroom tissue which is acceptable to consumers, it is necessary to provide a soft tissue which has sufficient dry tensile strength for normal use. In addition, it is necessary that the tissue is sufficiently dispersible for flushing in reasonable quantities in typical household toilets while providing a tissue with sufficient degradability to be accommodated in septic systems. Conventional bathroom tissue does not possess sufficient resistance to wet abrasion to be suitable for use premoistened without tending to pill or shred as described above.
Permanent wet tensile strength would normally interfere with both the dispersibility and degradability of the product and thus prevent the tissue from being compatible with a septic system. In addition, permanent wet tensile strength can often interfere with the flushing of the tissue in a typical household toilet either by clogging the bowl or being retained within the pipeline connecting the house to the sewer thus causing clogging, particularly, if roots are present to some extent as is often the case in older homes.
Conventionally, wet tensile strength is obtained in a paper product by adding a permanent wet strength resin or agent, such as the polyamide epichlorohydrin resins sold by Hercules under the trademark Kymene.RTM., to the paper furnish. At least two mechanisms have been postulated to account for the mechanism by which wet strength resins act. One holds that wet strength resins form covalent bonds between adjacent fibers while another holds that the wet strength resin places a layer over the hydrogen bonds formed between adjacent paper fibers and thus prevents water from breaking the hydrogen bonds. In a permanent wet strength product, the strengthening effect does not decay with time. Accordingly, paper products produced with permanent wet strength resins would not normally be acceptable for use in a conventional household toilet or for use with a septic system.
To provide temporary wet strength, specialized temporary wet strength resins are incorporated into a cellulosic web. The nature of the resin chosen does not seem to be particularly critical provided that it provides wet strength properties as described herein. Suitable products are usually water soluble polymers or monomers and oligomers capable of forming water soluble polymers. Typically, these resins are water soluble organic polymers comprising aldehydic units or alternatively aliphatic dialdehydes such as glyoxal and cationic units. It is thought that these polymers or aliphatic dialdehydes form hemiacetal linkages with the cellulose and that these hemiacetal linkages hydrolyze at a moderate rate when immersed in water, so tissues incorporating these resins have considerable initial wet strength but after only a few minutes, the wet strength drops to some suitably low value to make the tissue flushable. In practice, the initial wet strength of tissues made using these resins tends to increase moderately over the first several days subsequent to manufacture thereof. In our experience, wet strength tends to be fairly well leveled out within about a week after manufacture, so throughout this specification and claims, where we refer to wet strength, that wet strength should be understood to be wet strength as obtained after about a week of aging unless the context clearly indicates otherwise.
Usually, cleansing of the perineum and adjacent regions of the human body is performed with bathroom tissue in a dry condition. Dry tissue does not always cleanse these regions as thoroughly as may be desired. Some users would prefer to use a bidet to assist with the cleansing of these regions for a feeling of extra cleanliness. However, if an individual uses conventional bathroom tissue after the perineum and adjacent regions are thoroughly wet or proceeds to moisten the tissue prior to use of the tissue, known bath tissues, even those few brands having significant wet strength, have a tendency to pill.
Pilling is a phenomenon occurring during use wherein small balls of tissue cling either to the surface of the tissue or to the user, possibly leading the tissue to shred before cleaning is complete. Such a condition is not desirable to most users. One purpose of this invention is to provide a flushable, sewer and septic-compatible tissue product which may be moistened before use and still retain sufficient softness, strength and resistance to pilling to be used in cleaning.